1. Field of the Invention
This invention generally relates to a method and apparatus for coupling radiant energy from a radiant energy source to the conducting medium of a primary optical system. The primary optical system may be, but is not necessarily limited to, an optical fiber system.
The invention also relates, in applications where the primary optical system is a fiber optic conductor, to a fiber optic connector for coupling focused radiant energy from a laser to a fiber optic conductor, and to a method of coupling radiant energy from the laser to the conductor, in which radiant energy that fails to couple with the conductor is directed away from sensitive structures, and preferably (though not necessarily) deflected back to the laser system itself, by means of a secondary transmission path and one or more reflectors and/or heat sinks, so that the errant radiant energy can be safely dissipated.
By directing excess radiant energy in this manner, the excess radiant energy is prevented from damaging the connector and optical components in the connector, thereby enabling the use of smaller fibers with greater coupling tolerances, and more specifically permitting the use of optical fibers having core diameters smaller than the focused spot of the laser source, that are not precisely aligned with the focused spot, and/or that have an acceptance angle too small to accept all of the focused radiant energy.
Finally, the invention also relates to a fiber optic termination, and to a method of terminating an optical fiber, that reduces or eliminates launching energy into the cladding of the fiber.
2. Description of Related Art
The invention provides a solution to the problem of errant radiant energy when coupling radiant energy from lasers or other coherent radiant energy sources, which are typically but not necessarily monochromatic, to relatively small primary optical transmission systems, for example, optical fibers used in medical devices such as scalpels or lithotripter fibers. Such optical fibers are especially useful to implement recently developed, minimally invasive surgical techniques.
An example of an apparatus to which the principles of the invention may be applied is illustrated in FIGS. 1 and 2. The apparatus includes a laser system 12 and a standard connector coupler 18 for coupling a connector such as connector 30 shown in FIG. 2, which couples the output of the laser system to a primary optical system such as an optical fiber or fiber cable 32. The laser may be a high energy pulse or continuous wave laser that generates a monochromatic radiant energy output beam 17. For example, the laser system may be a Holmium:YAG laser that generates an output formed of pulses on the order of 250μ seconds in pulse width and energy levels ranging up to 1800 mj/pulse with an average power of 12 Watts. The output beam 17 is passed through a condensing lens to form an output beam 17a that is focused on a spot 17b in the vicinity of input focal plane 16 and centered in a connector coupler 18 mounted on the laser enclosure 15 using respective X, Y, and Z adjustments 12a-12c. When connector 30 shown in FIG. 2 is secured to connector coupler 18 by locking member 40 (which may, for example, be an internally threaded nut), the connector ferrule 31 is ideally centered on focused spot 17b and the distal end of the connector ferrule is at the focal plane 16. In this example, the focused spot size at the focal plane 16 is on the order of 365 microns and the relative power density at the focal plane for a 365 micron spot with an average power of 12 watts is approximately 11.5 kW/cm2. On the other hand, the power density 6 mm beyond the focal plane 16 is reduced by a factor of 50.
Ferrule 31 of connector 30 is typically a metal elongated hollow body member into which is inserted the optical fiber or fiber cable 32. The proximal end has a fiber clearance hole 38 drilled close to the outside diameter of the fiber 35. To secure the fiber 35 to the ferrule 31, a small portion of the fiber optic cable 32 is stripper away exposing the glass fiber 35. Before the stripped fiber is placed inside ferrule 31, an adhesive 34 may be applied to a small portion of exposed fiber 35 and the exposed fiber is passed through the internal diameter of the ferrule to its distal end. The extreme distal portion of the exposed fiber exits the ferrule through fiber clearance hole 38. Later, after adhesive 34 is cured, the exposed fiber 35 is trimmed and polished such that the distal end of the ferrule and the distal end of the fiber 35 are flush. Alternately, the fiber 35 may be secured within the ferrule by crimping a portion of the cable 32 to the ferrule using a sleeve.
The errant radiant energy problem arises when the primary optical transmission system approaches or is smaller than the size of the focused beam of radiant energy. For example, the smaller the diameter of an optical fiber, the more difficult it is to focus energy from the laser into the core. If the core diameter is smaller than that of the focused spot of the laser source, or if the focused radiant energy to the core is misaligned or greater than the fiber's acceptance angle, then energy will be transferred to structures that make up the coupler or that surround the core. The density is often great enough to soften, melt, or fuse any materials which are not highly optically transmissive or reflective. In many cases the energy density can be so great that photo thermal ablation may occur in the metal housing of the connector that couples the laser to the fiber, causing the metal to explosively form a plume mixture gases and micron size particles, which re-deposit and contaminate the focusing lens. Further lasing into the contamination can create extreme localized heating which ultimately destroys the focusing lens.
There is therefore a need for a coupling apparatus and method that minimizes the impact of radiant energy that fails to couple to the core of the optical fiber (or other primary optical transmission system).
Furthermore, the coupling apparatus and method must be compatible with existing laser systems and connectors, such as the one illustrated in FIG. 1. For example, in medical applications, connectors that implement the invention generally should be compatible with standard medical laser industry connectors, such as the SMA 905 standard connector.
Finally, even when all of the energy that fails to couple to the fiber is dissipated, a further problem arises in that some of the energy that couples to the fiber will couple to the cladding of the fiber rather than to the core, resulting in the problem that the cladding will act as a secondary wave guide and leak energy into surrounding coating during tight bends, such as my occur when the optical fiber is used for laser lithotripsy after it has been passed through the working channel of an endoscope. While the amount of coupling may be reduced by tapering, the core and cladding may mix, causing light to also mix into the cladding, and higher order modes may be created which are more subject to loss during a bend than lower order modes. By way of background, it was proposed in U.S. Pat. No. 6,282,349 to fuse the cladding to the ferrule in which it is placed, but the cladding fusion scheme described in this patent did not involve removal of some or all of the cladding at the end of the fiber reduce coupling of laser energy into the cladding.
There is therefore also a need for a laser-to-fiber coupling arrangement that reduces or eliminates coupling of focused radiant energy into the cladding of an optical fiber, rather than into the core.